The invention relates to a process for the production of exposed, holographic media containing a photopolymer formulation. The invention furthermore relates to an exposed holographic medium which is obtainable by the process according to the invention.
WO 2008/125229 A1 describes photopolymer formulations which can be used for the production of holographic media. The photopolymer formulations disclosed comprise polyurethane-based matrix polymers, acrylate-based writing monomers and photoinitiators. The holographic media obtainable with the aid of the photopolymer formulations are distinguished by high brightness, which is described physically by their high refractive index contrast (Δn).
In the known process, it is not possible to vary the mechanical properties of the exposed holographic media obtained on keeping Δn substantially constant. Thus on modification of these properties by changing the chemical composition, the Δn value also changes here. A variation of the mechanical properties without significant change in the refractive index contrast is, however, desirable in many cases since, depending on the field of use, holographic media have to meet very different requirements with regard to the mechanical properties. Thus, for example, media having a mechanical modulus of less than 0.7 MPa can be laminated with a substrate without additional adhesive. On the other hand, it may be desirable to obtain media which are insensitive to damage due to external influences. This can be ensured when the media have a high modulus in the region of 15 MPa or more.
Furthermore, particularly when holograms are used as a security feature, it is of decisive importance to be able to adjust the mechanical properties according to wishes without the Δn value significantly changing at the same time. Thus, it may be desirable in particular that an exposed holographic medium cannot be removed without destruction from a marked product, such as an identity document. Media with a flexible rubber-like consistency which have a correspondingly low modulus in the region of less than 0.7 MPa are suitable for this purpose. Conversely, however, it may also be necessary in particular to permit destruction-free transfer. In this case, the medium should have relatively high rigidity, which corresponds to a modulus in the region of 15 MPa or more. These different aspects cannot be realised by the process known in the prior art.